[Omp] Automated tuning of omp pragmas?

[Marcel Beemster]

Christian,

You wrote:
> > > I've noticed that OpenMP offers a number of pragmas with tunable
> > > parameters such as chunk-size.  I imagine that in a program with many
> > > OpenMP sections, this presents a very large tuning space.
> > >
> > > Has anyone looked at using an automated method (i.e. a stochastic
> > > search) to search through the tuning space to look for values that
> > > perform well on a given target system?

Can't help you directly, but I assume you refer to the
tuning of the "schedule" clause as described in Table 2.3 on
page 36 of the OpenMP spec.

On the surface it appears to me that for each parallel
section the schedule clause is independent (ignoring nested
parallelism). In that case the parameters of the search
space are independent and the search space is not that big.
You can tick off the parallel sections one by one, keeping
complexity low.

However on an architecture where cacheing has a large impact
on the performance you have to look at the data
mapping between the iteration space and the individual
processors. Preferrably you want to keep the data allocation
similar from one parallel section to the next. This implies
a coupling between adjacent (at run-time) parallel sections
and then indeed the parameters are not independent any more.

In my experience, it helps to have as much as possible
information as input for an optimization algorithm. In this
case the facts that adjacent parallel sections are coupled
and that there is usually, though not necessarily, a fixed
mapping from the iteration space (the i parameter) to the
data allocation on the processors.

Other than that, any optimization algorithm can be used, it
does not have to be OpenMP specific.

Best,
	Marcel

-- 
Dr. Marcel Beemster, Senior Software Engineer, marcel at ace.nl,www.ace.nl
Associated Compiler Experts bv. Amsterdam, Netherlands. +31 20 6646416.
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